The present invention relates to a method and apparatus for determining a performance characteristic of a fan, and in particular, though not limited to a method and apparatus for determining the speed of a cooling fan in an enclosure for electronic circuitry.
Computers comprise a cabinet within which the electronic components of the computer are housed. Some of these components, for example, the central processing unit and the power supply module operate at relatively high temperatures, thus generating significant quantities of heat. One or more cooling fans are provided in such computer cabinets for circulating air and for changing air within the computer cabinet for maintaining the temperature within the cabinet within an acceptable temperature range. In order to minimise acoustic noise and power consumption such fans are only operated when the temperature exceeds an upper limit of the acceptable temperature range. More recently variable speed control techniques have been developed for operating the fans at various speeds, depending on the temperature within the computer cabinet. One of the more recent variable speed control techniques which have been developed is based on pulse width modulation of the power supply to the fan motor. Typically, a control circuit outputs a pulse width modulated signal for switching the power supply to the fan motor so that the power supply to the fan motor is pulse width modulated. The control circuit also measures the speed of the fan by monitoring a tachometer signal received from the fan motor. However, one problem with such fan control is that in general the tachometer signal from the fan motor is only available when the duty cycle of the pulse width modulated power supply to the fan motor is in the on state. Thus, the tachometer signal read over one or more duty cycles is not an accurate representation of the fan speed.
Due to the relatively high heat output of some central processing units and power supply modules, it is essential that the cooling fans should operate at a speed which is at least up to the speed at which they are set to operate, and which is determined by the mark/space ratio of the duty cycle of the pulse width modulated power supply. An under-speed in a fan can have serious consequences including failure and destruction of components within the computer cabinetry. It is therefore essential that under-speed in a fan can be accurately detected and determined.
The present invention is directed towards providing a method and apparatus for determining a value of a performance characteristic of a fan, such as for example fan speed.
According to the invention there is provided a method for determining a value of a performance characteristic of a fan driven by a motor powered by a pulse width modulated power supply in which a monitoring means associated with the fan outputs a signal indicative of the value of the performance characteristic to be monitored and the monitoring means is powered by the pulse width modulated power supply, the method comprising the steps of:
reading a predetermined portion of the signal from the monitoring means during a period of a duty cycle of the pulse width modulated power supply when the power supply during the duty cycle is in the on state for powering the fan motor,
holding the power supply in the on state during the duty cycle while the predetermined portion of the monitoring means signal is being read, and
determining the value of the performance characteristic from the read predetermined portion of the monitoring means signal.
In one embodiment of the invention the off period of the next duty cycle of the power supply to the fan motor is commenced in response to the predetermined portion of the monitoring means signal having been read, in the event that the power supply had been held in the on state longer than the normal on period of the duty cycle.
In another embodiment of the invention the performance characteristic to be determined is the fan speed, and the monitoring means outputs a pulsed signal indicative of the fan speed, and a predetermined number of sequential pulses of the pulsed signal are read, and the time period during the predetermined number of sequential pulses of the pulsed signal is determined for determining the time period of one revolution of the fan motor.
In a further embodiment of the invention the predetermined number of sequential pulses of the pulsed signal from the monitoring means which are read is at least two sequential pulses. Preferably, the predetermined number of sequential pulses of the pulsed signal from the monitoring means which are read is at least three sequential pulses.
In one embodiment of the invention the time period during the predetermined number of sequential pulses of the pulsed signal from the monitoring means which are read is determined by counting the number of clock pulses in a reference clock signal during the time period for the predetermined number of sequential pulses. Preferably, the number of clock pulses of the reference clock signal are counted from an edge of the first of the sequential pulses of the pulsed signal to a corresponding edge of the last of the sequential pulses of the predetermined number of sequential pulses for determining the time period during the predetermined number of sequential pulses of the pulsed signal from the monitoring means.
In one embodiment of the invention the pulsed signal from the monitoring means is derived by monitoring the current through the fan.
Preferably, the value of the performance characteristic determined from the signal from the monitoring means is compared with a reference value of the performance characteristic at which the fan should have been performing in response to the duty cycle of the pulse width modulated power supply for determining if the fan is operating at the reference performance characteristic value, and an alert signal is issued in the event that the determined value does not comparing favourably with the reference value of the performance characteristic. Advantageously, the alert signal is issued in the event of the determined value of the performance characteristic being less than the reference value of the performance characteristic.
In one embodiment of the invention the method according to the invention is for monitoring the fan speed of a cooling fan located in an enclosure for electronic circuitry.
Additionally, the invention provides apparatus for determining a value of a performance characteristic of a fan driven by a motor powered by a pulse width modulated power supply in which a monitoring means associated with the fan outputs a signal indicative of the value of the performance characteristic to be monitored, and the monitoring means is powered by the pulse width modulated power supply, the apparatus comprising:
a means for reading a predetermined portion of the signal from the monitoring means during a period of a duty cycle of the pulse width modulated power supply when the power supply during the duty cycle is in the on state, for powering the fan motor,
a control means for holding the power supply in the on state during the duty cycle while the predetermined portion of the monitoring means signal is being read, and
a means for determining the value of the performance characteristic from the read predetermined portion of the monitoring means signal.
In one embodiment of the invention the control means holds the power supply in the on state during the duty cycle in response to the reading means commencing to read the monitoring means signal.
In another embodiment of the invention the control means is responsive to the predetermined portion of the monitoring means signal having been read for commencing the off period of the next duty cycle of the power supply to the fan in the event that the power supply had been held in the on state for a period longer than the normal on period of the duty cycle.
In a further embodiment of the invention the performance characteristic to be determined is the fan speed, and the monitoring means outputs a pulsed signal indicative of the fan speed, and the reading means reads a predetermined number of sequential pulses of the pulsed signal, and a computing means determines the time period during the predetermined number of sequential pulses of the pulsed signal for determining the time period of one revolution of the fan motor.
In one embodiment of the invention the predetermined number of sequential pulses of the pulsed signals from the monitoring means read by the reading means is at least two sequential pulses. Preferably, the predetermined number of sequential pulses of the pulsed signal from the monitoring means read by the reading means is at least three sequential pulses.
In a further embodiment of the invention the computing means for determining the time period during the predetermined number of sequential pulses from the monitoring means comprises a counting means for counting the number of clock pulses of a reference clock signal during the time period for the predetermined number of sequential pulses of the pulsed signal from the monitoring means.
In one embodiment of the invention the counting means counts the number of clock pulses of the reference clock signal from an edge of the first of the sequential pulses of the pulsed signal to a corresponding edge of the last of the sequential pulses of the predetermined number of sequential pulses of the pulsed signal from the monitoring means.
Advantageously, the reading means is responsive to at least one pulse of the pulsed signal from the monitoring means having been received after commencement of the on period of the duty cycle of the pulse width modulated power supply prior to commencing timing of the predetermined number of sequential pulses.
Preferably, the reading means is responsive to the second pulse of the pulsed signal from the monitoring means having been received after the commencement of the on period of the duty cycle of the pulse width modulated power supply for commencing timing of the predetermined number of sequential pulses.
In one embodiment of the invention a comparing means is provided for comparing the value of the performance characteristic determined from the signals from the monitoring means with a reference value of the performance characteristic at which the fan should have been performing in response to the duty cycle of the pulse width modulated power supply for determining if the fan is operating at the reference performance characteristic value, and a means for outputting an alert signal is also provided which is responsive to the comparing means determining that the determined value of the performance characteristic does not compare favourably with the reference value of the performance characteristic. Preferably, the alerting means is responsive to the comparing means determining that the determined value of the performance characteristic is less than the reference value of the performance characteristic.
In a further embodiment of the invention the apparatus further comprising the monitoring means, and the pulsed signal outputted by the monitoring means is derived by the monitoring means from the current through the fan motor.
In one embodiment of the invention the monitoring means comprises a monitoring resistor in series with the fan motor, and the pulsed signal is a pulsed voltage signal developed across the monitoring resistor resulting from spikes in the current through the fan motor.
In another embodiment of the invention the monitoring resistor is coupled to the reading means by a coupling capacitor.
In a still further embodiment of the invention the apparatus is for monitoring the fan speed of a cooling fan located in an enclosure for electronic circuitry.
The advantages of the invention are many. By virtue of the fact that the power supply to the fan motor is held in the on state during a duty cycle of the pulse width modulated power supply while the predetermined portion of the signal from the monitoring means is being read, the value of the performance characteristic being determined can be determined as accurately as possible. This is due to the fact that the signal from the monitoring means gives a representation of the value of the characteristic which is as accurate as possible, since the monitoring means remains powered continuously during the period while the signal from the monitoring means is being read. Additionally, by holding the power supply in the on state during the duty cycle of the pulse width modulated power supply while the predetermined portion of the monitoring means signal is being read, allows the fan to be operated and the fan speed to be determined by a two wire system remotely or otherwise, assuming a common ground is provided. All that is required is one wire for modulating the width of the pulse width modulated power supply to the fan motor, and a second wire for receiving the signal from the monitoring means. Additionally, the method and apparatus according to the invention provides a relatively simple and straightforward method and apparatus for accurately determining a value of a characteristic of a fan where both the fan and the monitoring means are powered by the same pulse width modulated power supply. The method and apparatus are particularly suitable for controlling the operation and determining the speed of a cooling fan in an electronic enclosure, for example, a computer cabinet for cooling components within the computer cabinet. Additionally, by virtue of the fact that the fan can be operated and the fan speed can be determined by the use of two wires only, the method and apparatus requires minimum additional circuitry.
The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawings.